Due to safety considerations, regulatory requirements for T5 and smaller diameter fluorescent lamps require that the lamp output power be limited in the case of end of lamp life (EOL) events. For example, at EOL one of the lamp filaments may cease to significantly emit electrons thereby causing the lamp to conduct in one direction but not in the opposite direction. During this condition, the lamp is said to be rectifying and may present a fault or hazard condition due to a high voltage.
A known method of providing EOL protection for a rectifying lamp is to include a detection capacitor in series with the lamps, and to sense the direct current (DC) voltage across the detection capacitor. In normal operation, the voltage across the detection capacitor will be near zero because the lamp currents are equal in both polarities. At EOL, if the lamp is rectifying, a DC voltage will accumulate across the detection capacitor, and the EOL condition can be sensed via the DC voltage. However, other fault conditions for a fluorescent lamp may result in no lamp current conduction in either direction, such as when the lamp glass envelope is cracked and the atmosphere of the lamp is lost. In this case, the rectification detection capacitor discussed above will not detect the fault. Accordingly, other methods must be used to detect the non-conducting lamp fault.
Therefore, there remains a need for a system and method that recognizes and mitigates fault and hazard conditions including lamp rectification and lamp open circuit conditions in ballasts that utilize closed loop feedback control of lamp current. There also remains a need for a system and method that recognizes and mitigates lamp faults as an open loop state in a closed loop control when a closed loop state is expected.